In recent years, an LED (light-emitting diode) or an organic EL (Electro-Luminescence) element is used as a backlight for a lighting apparatus or a liquid crystal panel. In order for a light-emitting element, such as the LED or organic EL element, to emit light of stable brightness, it is necessary to supply a constant current proportional to the brightness of the emitted light.
For supplying power to such light-emitting elements, a driving circuit with a DC/DC converter (switching regulator) may be utilized. The driving circuit of the related art turns off a switching transistor, when a driving current flowing through the LED reaches a predetermined peak level while the switching transistor is in an ON state. After a predetermined period of time, the switching transistor is turned on again. By repeating this operation, the driving current stabilized at the predetermined level is supplied to the LED.
In such driving circuits, a problem may occur with the operating frequency of the DC/DC converter changing, since on-time of a switching transistor undergoes a lot of changes in response to an input voltage of the DC/DC converter, i.e., power supply voltage. Therefore, when designing a lighting apparatus, there is a need to consider the variation of the operating frequency, which raises the difficulty level of designing such devices.